Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Making named article
Reexamination Certificate
2001-08-24
2004-05-11
McPherson, John A. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Making named article
C428S001260, C349S124000, C522S167000
Reexamination Certificate
active
06733958
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a photo-alignment layer used in a liquid crystal display device and, more particularly, to a material for forming a photo-alignment layer, capable of aligning liquid crystal molecules by light exposure, without being subject to a rubbing treatment, a photo-alignment layer made of the material, a method of manufacturing the same, and a liquid crystal display device using the photo-alignment layer.
2. Description of Related Art
In the liquid crystal display device, the state of molecular alignment of liquid crystals is changed by the action of an electric field and a change in optical characteristics attending the change is utilized for display.
In general, liquid crystals are used while interposed in the space between the two substrates. To align liquid crystal molecules in a specific orientation, the interior surfaces of the substrates are subjected to an alignment treatment.
The alignment treatment is usually performed by a rubbing method in which a layer made of a polymer such as a polyimide is formed on a substrate made of such as glass and then rubbed with a cloth in one direction. Consequently, liquid crystal molecules near the substrate are aligned so that their directors become parallel to the rubbing direction. For example, in a twist nematic (TN) cell, two substrates coated with an alignment layer on their interior surfaces face each other between two polarizers whose polarization directions are perpendicular to each other, and the substrates are further arranged so that their rubbing directions are parallel to the polarization direction, thereby making it possible to form a display which operates based on changes in light transmittance.
Although the rubbing method has the advantage that the manufacturing apparatus has a simple structure, a cleaning process is required after the alignment treatment because dust may adhere during the manufacturing process. Also in TFT liquid crystal cells, which have recently been widely used, TFT previously deposited on the substrate may be destroyed by static electricity which is generated at the rubbing process, thus lowering the manufacturing yield. In the liquid crystal display device, since the inclination of constituent liquid crystal molecules has some directionality, problems arise, such as viewing-angle dependence in which the display color or contrast varies depending on the viewing angle.
As one method of solving the problem, for example, a multi-domain method of providing different pretilt angles (Japanese Unexamined Patent Application, First Publication No. Sho 62-159119) or alignment directions (Japanese Unexamined Patent Application, First Publication No. Sho 63-106624) of the liquid crystal molecules for different regions obtained by dividing each pixel, has been proposed. Such multi-domain is not suited for rubbing alignment method because of the complicated process required.
To solve these problems, there has recently been interest in liquid crystal alignment controlling techniques which do not use rubbing. As non-rubbing alignment techniques, for example, the oblique evaporation, LB (Langmuir Blodgett's) layer, photolithography and photo-alignment methods have been studied. Among these methods, the photo-alignment method, in which a coating layer formed on a substrate is exposed to polarized light to provide a liquid crystal aligning property, is simple and does not require a cleaning process after the alignment treatment, and multi-domain can also easily be performed by using such as photo-mask method. Therefore, the photo-alignment method has been intensively studied. As the photo-alignment method, a method utilizing photoisomerization of a photo-alignment moiety (e.g. an azo group) capable of carrying out a photo-alignment function in organic molecules, a method utilizing photo dimerization of a cinnamoyl group, a coumarin group or a chalcone group, a method utilizing photocrosslinking of a benzophenone group, and a method utilizing photodegradation of a polyimide resin have been reported.
As the material for the photo-alignment layer, which utilizes photoisomerization, photo dimerization or photocrosslinking, a polymeric material is often used to obtain a uniform layer when coated on a substrate made of such as glass, and the photo-alignment moiety is often introduced into the side chain or the main chain of the polymeric material. It is also possible to use a material prepared by using molecules having a photo-alignment property as guest molecules and dispersing them in a host compound consisting of a polymeric compound.
However, the photoisomerized materials is inferior in photostability after the photo-alignment treatment because it utilizes cis-trans photoisomerization of molecules by exposure to polarized UV. In the case of photo-degradated materials, since the liquid crystal is contaminated with the degradation product produced during the photo-alignment treatment, the substrate must be cleaned after the treatment, thereby losing the feature that no cleaning of the photo-alignment layer is required. Further, almost all photo-alignment materials using the polymeric material have the problem that the kind of a solvent used when coated on the substrate is limited because of their poor solubility in solvents.
For example, WO9637807 (U.S. Pat. No. 6,001,277, Japanese Unexamined Patent Application, First Publication No. Hei 8-328005) discloses a liquid crystal layer using a resin having a photoisomerizable structural unit, which exhibits dichroizm, and a reactive functional group. This material is a polymeric compound and the kind of the solvent used when coated on the substrate is limited, and a high-boiling point polar solvent such as N,N-dimethylacetamide or N-methyl-2-pyrolidone is used. In this case, a long time is required to evaporate the solvent after coating, thereby lowering the productivity. Furthermore, this material has a low crosslink density because of the small proportion of reactive functional groups in the resin. As a result, the heat resistance of the alignment layer made of this material is not always sufficient.
Examples of the method to solve these problems, thereby achieving a durable and stable liquid crystal aligning property of the photo-alignment layer include a method of thermally polymerizing or photopolymerizing a polymerizable monomer having a photo-alignment moiety, which shows an alignment property upon exposure to polarized light, and photo-aligning the resulting polymer by exposure to polarized light. In general, however, it becomes necessary to add a polymerization initiator to thermally polymerize or photopolymerize the monomer. Since this polymerization initiator is a low-molecular weight compound, the polymerization initiator diffuses into the liquid crystal in the cell with the lapse of long periods of time and thus the characteristics of the liquid crystal display device (e.g. voltage holding ratio) are likely to be deteriorated.
A photopolymerizable group which does not require a polymerization initiator is a polymerizable maleimide group. A photo-alignment layer using a compound having this polymerizable maleimide group is disclosed in Japanese Unexamined Patent Application, First Publication No. 2000-53766 (U.S. Pat. No. 6,218,501) and Japanese Patent No. 2962473 (Japanese Unexamined Patent Application, First Publication No. Hei 11-2815, U.S. Pat. No. 6,048,928). Although in these photo-alignment layers, a functional group capable of inducing photo-alignment is added as a side chain to the main chain of polymaleimide, these photo-alignment layers are still insufficient in the long-term stability of their heat resistance and liquid crystal alignment capability.
BRIEF SUMMARY OF THE INVENTION
An object to be achieved by the present invention is to provide a photo-alignment layer for liquid crystal display devices, which has good liquid crystal display device characteristics such as a high voltage holding ratio and also has good alignment stability and sufficient resistance to lig
Fukuda Masanobu
Hayakawa Hitoshi
Takada Hirokazu
Armstrong Kratz Quintos Hanson & Brooks, LLP
Dainippon Ink and Chemicals Inc.
McPherson John A.
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